This invention generally relates to an equalizer for equalizing the amplitude frequency characteristics of a transmission system, and more particularly to an equalizer having excellent amplitude and phase characteristics for high frequency bands in excess of several hundred MHz, which characteristics at such high frequency have not been obtained by concentrated constant circuit elements.
It has been known that, in a long-distance coaxial transmission line, the line loss generally increases with frequency. Thus, equalizers are inserted into suitable portions of such a transmission line for flattening the amplitude-frequency characteristics thereof, and it is desirable to make the equalizer controllable.
A typical equalizer of conventional construction is shown in FIG. 1. As illustrated in the figure, the equalizer is formed of a bridged constant resistance bridged-T-type circuit, and constituent elements Z.sub.1 and Z.sub.2 are formed of concentrated circuit constants or distributed circuit constants, depending on the frequency bands wherein they are used.
If the elements Z.sub.1 and Z.sub.2 are distributed constant circuits, the equalizer can be used in high frequency bands, but the element Z.sub.1 should have a construction of a dual coaxial line since it floats with respect to the ground. See "A constant resistance bridged-T equalizer using transmission-line elements" by Raymond A Thatch IEEE Transactions on circuit theory, September 1973 pp 577-580.) Accordingly, the frequency range wherein such an equalizer can be used is restricted to several hundred MHz at most.